The invention relates to detectors used in scanning electron microscopy.
In scanning electron microscopy, an electron beam bombards a sample and produces three signals: secondary electrons (ejected from very near the surface of the sample with low energy), backscattered electrons (originating deeper in the sample with higher energy), and x-rays. One or more of these signals are detected as the electron beam is moved by a magnetic lens across a small area of the sample to create an enlarged image. Most commercial scanning electron microscopes (SEMs) are provided with one or more detectors essentially dedicated to measuring one of these three signals at a time. Newer SEMs are capable of mixing signals from backscattered and secondary electrons, but still use two separate detectors. In these cases, typically one detector (usually the backscatter detector) is placed directly over the sample, and the other (secondary electron detector) is located off to one side of the sample.
Ring-shaped scintillator detectors have been employed in devices where the electron beam passes through the hole of the ring in its travel to the sample. Microchannel plates (MCPs) have also been used as SEM detectors; an advantage of MCPs is that they are very sensitive to low energy electrons, and low energy electron beams can be used.
Helbig, H. F. et al., "Channel Plate Detection in Low Energy Scanning Electron Microscopy", Scanning Microscopy. Vol. 1, No. 4 (1987) pp. 1491-1499 discloses using an MCP (actually two plates connected electrically in series and joined together to form a single resistive element) with a 6 mm center hole as a low profile SEM detector, the electron beam of the SEM passing through the hole, the emitted electrons being detected at the lower, facing surface of the MCP.
Venables et al. U.S. Pat. No. 3,896,308 discloses a detector for an SEM that employs an MCP having a hole through it for passage of an electron beam.
Kruit, P. and Dubbeldam, L., "An Electron Beam Tester with Dispersive Secondary Electron Energy Analyzer", Scanning Microscopy. Vol. 1, No. 4, (1987), pp. 1641-1646, discloses using two scintillator detectors to detect backscattered and secondary electrons that are attracted by the magnetic field used in the lens and travel back through the SEM lens to the detectors.